One of the most critical steps in the wet-processing of semiconductor device wafers is the drying of the wafers. An ideal drying process would leave absolutely no contaminants on the wafer surfaces, while operating quickly, safely, and with no environmental or safety risks. In practice, deionized (DI) water is most frequently used as the process liquid. Most liquids such as DI water will "cling" to wafer surfaces in sheets or droplets due to surface tension following extraction of the wafer from a liquid bath. In other words, the liquid will "wet" the solid surface so long as the adhesion of liquid molecules on the surface of the solid is greater than the cohesion of the liquid molecules.
Changing the phase of the process liquid to gas (vapor) phase reduces the drying problems inherently caused by surface tension. Phase transitions, however, have long been acknowledged as having a high probability of allowing contaminants entrained in the surface boundary layer of the liquid to deposit on and adhere to the wafer surface, resulting in a higher rate of defects in the end product electronic devices. Various technologies have been developed in an attempt to control the phase transition, to reduce the level of contaminants left on the wafer surface after drying.
The following drying technologies have been used in the past:
A. Hot Water Dryers. These dryers operate quite simply. Wafers are immersed in a hot bath of DI water, and as they are slowly withdrawn from the bath, the water retained on the wafer surface due to surface tension is evaporated from the heated silicon surface. However, hot DI water attacks silicon surfaces, rendering this technology largely useless for the fabrication of most semiconductor devices.
B. Spin-Rinse Dryers. These dryers operate on two fundamental mechanisms. First, bulk liquid is removed from the wafer surface by spinning the wafer and generating centrifugal force. Once the bulk liquid has been removed, surface tension between the substrate and the residual liquid is greater than the level of centrifugal force which can be reasonably applied to the wafer. However, a second mechanism, evaporation, also comes into play. The evaporation rate is commonly increased by maintaining a relatively high rotational velocity on the wafer, thus improving convection. Heated nitrogen gas (N2) is typically injected into the process chamber to further increase evaporative drying.
This drying technology is limited, however, by the following factors: (1) it is unsuitable for drying hydrophobic surfaces, as minute water drops become isolated on the water surface and are difficult to remove, and contaminants entrained in such droplets are deposited on the wafer surface; (2) high spin velocity, which improves drying, generates high turbulence which can cause contaminants to deposit on the wafer surface; and (3) high mechanical stresses can be generated by the forces created from the high spin velocities, causing damage to the wafers, or generating contaminant particles.
C. Isopropyl Alcohol (IPA) Vapor Dryers. These dryers operate by immersing wafers wetted with DI water into a heated environment saturated with IPA vapor. Liquid IPA has a significantly lower surface tension than that of water. As IPA starts to condense on the wafer surface, water which was present on this surface is displaced by IPA. When the water has been displaced by IPA, the wafers are then withdrawn through a cool zone which completes the condensation of the alcohol and causes it to flow off of the wafer surface.
This drying technology is limited by the following factors: (1) it involves the inherent hazard of using IPA, a flammable liquid, to be boiled at a temperature well in excess of its flash point; (2) it requires the consumption of IPA at relatively high rate; (3) it creates relatively high fugitive organic vapor emissions.
D. Marangoni Dryers. These dryers essentially create an alcohol enriched interface at surface of the rinse liquid. As the wafers are withdrawn through the interface, the alcohol helps to displace water, reduces surface tension on the water surface, and allows water to be "pulled" from the wafer surface through cohesive attraction with the bulk liquid. This technology, however, also have fugitive organic emission problems inherent with any drying process that requires the utilization of alcohol.
E. Bran, U.S. Pat. No. 5,556,479, discloses a wafer drying process involving the slow draining of a rinsing fluid from a processing tank while heating portions of the wafer surfaces which are in contact with a fluid interface as air or another gas replaces the process fluid. In this process, the wafer is heated at the fluid interface up to a sufficient temperature to produce convection currents in the process fluid. The wafer is preferably displaced from the rinsing fluid at a rate no faster than 7 or 8 centimeters per minute. Consequently, the manufacturing throughput rates with this technique are lower than desired.
In light of the limitations inherent to these and other drying processes, it is an object of the present invention to provide a novel process and apparatus for drying semiconductor wafers or similar items quickly and safely while leaving minimal levels of particle contaminants or chemical residue.
It is a further object of the invention to accomplish such drying while reducing the hazards and emissions associated with drying wafers or similar items using chemicals.